The cause of his possible canonization was the subject of a good deal of political maneuvering on the part of his cousin, Charles V of France, who endorsed it, and his rival, John of Montfort, who opposed it.

[2] It is said that he placed pebbles in his shoes, slept on straw instead of a bed, confessed every night in fear of sleeping in a state of sin, and wore a cilice under his armor in battle.

When Charles asked the squire to take part in the prayer, the younger man refused, saying that the men who were buried at the English graveyards had killed his parents and friends and burned their houses.

[6] Upon returning to France, he decided to travel barefoot in winter from La Roche-Derrien to Tréguier Cathedral out of devotion to Saint Ivo of Kermartin.

[6] Charles was eventually killed in combat during the Battle of Auray in 1364, which with the second treaty of Guerande in 1381 determined the end of the Breton War of Succession as a victory for the Montforts.

[6] By his marriage to Joan the Lame, Countess of Penthièvre, he had five children: According to Froissart's Chronicles, Charles also had an illegitimate child, John of Blois, who died in the Battle of Auray.

Such variety of ex votos bedecked his tomb, that in 1368 Duke John IV of Brittany persuaded Pope Urban V to issue a bull directing the Breton bishops to stop this.